In the recent past, industrial robotic devices have played an increasing and more pivotal role in the manufacture of commercial products. These robotic actuator devices can typically be classified into either linear-type actuators or rotary-type actuators, both of which are generally constructed as rigid mechanical structures generating substantial forces and/or torque. These industrial devices, however, are often not suitable for use in biorobotics due to their non-natural compliance of robotic movement, as compared to natural human movement.
Biorobotic actuator devices which have been found suitable for use with, or as a replacement of, biological musculo-skeletal anatomies often include rigid skeletal structures moved by flexible artificial muscle actuators constructed to mimic the form and function of the biological components of real animals or humans. The artificial muscle, therefore, must be designed to function even when laterally deformed, and to include exceptional volumetric efficiency for the amount of linear displacement produced.
Rotary-type actuators, which transmits energy by applying a torque to a shaft rotating about a longitudinal axis thereof, are typically difficult to incorporate as artificial muscle replacements. The electric motors employed necessitate the application of additional conversion mechanisms to convert rotary motion into useable linear motion. These conversion mechanisms, such as linkages, cams, gears, pulleys, etc., become very cumbersome to arrange when attempting to apply these actuators to prosthetic devices which often require that many actuators fit into a small deformable volume while maintaining the high volumetric functional efficiencies of biological musculo-skeletal systems. One such patented system, however, is disclosed in U.S. Pat. No. 4,843,921 to Kremer.
Hydraulic cylinder actuators, by comparison, may be better adapted to mimic biological muscle since both generate a linear force and thus a linear motion. Generally, the outward pressure urged outwardly upon on the cylinder walls is converted into an axial force urging the piston into or out of the chamber. One substantial problem associated with hydraulic cylinders is that they must be substantially rigid since a fluid tight seal must be formed between the cylinder walls and the opposed surface of the inner piston. These small clearances, however, are difficult to maintain for flexible materials. Therefore, conventional hydraulic cylinders are usually substantially rigid structures which oppose substantial deformation and thus lack pliability of biological muscles. Compared to real muscle tissue which can and does operate when laterally deformed, the rigid physical property of hydraulic cylinder actuators limit their application in duplicating biological anatomy.
To address these deficiencies, several artificial muscle assemblies have been developed in the recent past which produce linear displacement and are flexible in nature. The most well renown is the McKibben Artificial Muscle, developed by Dr. Joseph McKibben, in the 1950's for use in an arm prosthesis. Briefly, this design employs an elongated, expandable inner bladder positioned inside a larger diameter braided or woven tube having strategically oriented fiber filaments. This woven tube arrangement enables a controlled radial expansion of the expandable bladder, when pressurized, which causes the opposed ends to axially contract. Thus, the overall longitudinal dimension of the artificial muscle contracts to produce the linear displacement relative the opposed ends of the inner bladder and woven tube.
The primary problem associated with this design is that the bladder and tube combination is only capable of contracting about thirty (30) percent of its rest length. This relatively small linear displacement substantially limits its use in biomechanical systems since the joint dimensions, as well as the tendon attachment and routing, become very critical. In addition to substandard joint geometry and/or tendon routing, other factors may substantially affect the range of motion of the joint such as tendon stretching and mechanical wear. Typical of the basic McKibben artificial muscle design is disclosed in U.S. Pat. Nos. 5,474,485 to Smrt; 5,351,602 to Monroe; 5,185,932 and 5,021,064 to Caines; and 4,739,692 to Wassam et al.
Finally, hydrogels (pH muscles) are also presently being developed as a means for artificial muscle. These hydrogel muscles have several characteristics similar to human muscle, and may change in volume by as much as 1000% when the pH is altered. The present designs, however, are relatively slow to operate and currently produce much smaller linear forces than would be operationally feasible. Moreover, hydrogel muscles are acid based which increases the difficulty in handling, transport and operation.